Plant composition dynamics following non-native ungulate removal: Convergence, divergence, and novel ecosystems

Global changes in disturbance regimes are reshaping ecosystems, driving shifts in species composition, diversity, and community structure. On islands, these effects are often pronounced due to their unique ecological contexts, including high levels of endemism and vulnerability to invasive species. Using three decades of longitudinal data, we examined vegetation dynamics on Santa Cruz Island, California (SCI), following the removal of feral ungulates, focusing on the interplay of convergence, divergence, and hierarchical complexity in community assembly. Specifically, we asked: (1) To what degree has species composition diverged within communities since ungulate removal? (2) Is there evidence of convergence in species composition among vegetation communities? Our analyses revealed patterns of divergence and convergence influenced by historical grazing intensity and local site variability. Divergence was most pronounced in grassland and fennel-dominated communities, where invasive species continued to dominate or alternate trajectories emerged. Conversely, convergence was observed among woody communities (e.g., coastal scrub, chaparral), characterized by increases in native shrub and tree cover. These shifts demonstrate the role of hierarchical complexity in ecological recovery, with local-scale processes such as competition and priority effects interacting with larger scale drivers like climate variability and disturbance legacies. Novel and hybrid ecosystems emerged in many areas, reflecting the interplay between native and invasive species because of a history of extreme disturbance. These findings demonstrate the challenges of managing ecological recovery in systems influenced by multiple perturbations. As global pressures on ecosystems increase, understanding the hierarchical dynamics of convergence and divergence offers critical insights for setting realistic conservation goals and managing biodiversity in recovering landscapes.